2023 Hypoxia Season Quick Facts

Background

Hypoxia is a condition of low dissolved oxygen concentrations in the waters of Long Island Sound that impacts up to half of the Sound’s waters each summer. The primary cause of hypoxia is nitrogen enrichment that comes from a variety of sources throughout the Long Island Sound watershed, in particular sewage treatment plants, stormwater runoff and atmospheric deposition.

Since 1991, the Connecticut Department of Energy and Environmental Protection (CT DEEP) and the Interstate Environmental Commission (IEC) have conducted an intensive water quality monitoring program on Long Island Sound. The program is funded through a grant from the EPA’s Long Island Sound Study. Data from the surveys are used to quantify and identify annual trends and differences in water quality parameters relevant to hypoxia (low dissolved oxygen), especially nutrients, temperature, and chlorophyll. These data are also used to evaluate the effectiveness of the management program to reduce nitrogen concentrations, since nitrogen is a primary contributor to the excessive algae growth that leads to hypoxia in LIS.

During the summer (June -September), surveys are conducted at bi-weekly intervals to better define the areal extent and duration of hypoxia. During these surveys stations are sampled for in situ parameters including dissolved oxygen, temperature, pH, and salinity.

The table below provides a summary of the 2023 Hypoxia Season at a glance. Data from all CTDEEP cruises can be downloaded from the UCONN ERDDAP. All samples are collected and analyzed under EPA-approved Quality Assurance Project Plans.

2023 Long Island Sound Hypoxia Season Cruise Information
Cruise Start Date End Date Number of Stations Sampled Number of Hypoxic Stations Hypoxic Area (mi2) Minimum DO Station Where Minimum DO Occurred
WQJUN23 05-31-2023 06-02-2023 17 0 0 6.24 B3
HYJUN23 06-23-2023 06-23-2023 18 0 0 6.38 A4
IEC RUN #1 06-28-2023 06-28-2023 22 0 NC 4.22 H-C1
WQJUL23 07-05-2023 07-10-2023 41 0 0 3.41 A4
IEC RUN #2 07-05-2023 07-05-2023 22 4 NC 1.87 9-413
IEC RUN #3 07-12-2023 07-12-2023 22 4 NC 2.25 H-C1
HYJUL23 07-17-2023 07-19-2023 40 3 42.01 1.85 A4
IEC RUN #4 07-18-2023 07-18-2023 22 12 NC 1.30 9-413
IEC RUN #5 07-25-2023 07-25-2023 22 6 NC 0.63 A5
WQAUG23 07-31-2023 08-03-2023 42 13 126.77 1.69 A4
IEC RUN #6 08-03-2023 08-03-2023 21 8 NC 0.97 H-D
IEC RUN #7 08-09-2023 08-09-2023 22 4 NC 2.57 H-B
IEC RUN #8 08-15-2023 08-15-2023 22 12 NC 1.37 B4
HYAUG23 08-14-2023 08-16-2023 38 8 136.72 1.94 A4
IEC RUN #9 08-22-2023 08-22-2023 22 0 NC 3.17 A3
IEC RUN #10 08-31-2023 08-31-2023 21 4 NC 2.23 H-B
WQSEP23 08-29-2023 09-07-2023 42 0 0 3.56 A4
IEC RUN #11 09-06-2023 09-06-2023 21 0 NC 4.14 H-C
HYSEP23 No Survey Conducted- DO > 3.0 mg/L NA NA NA NA NA NA
IEC RUN #12 09-12-2023 09-12-2023 22 14 NC 1.17 9-409

Click on the Tabs to explore more details.

Maximum Hypoxic Area

The maximum area of Long Island Sound with bottom water dissolved oxygen (DO) concentrations below 3.0 mg/L was 126.8 sq. mi (328.4 sq. km) and occurred during the WQAUG23 survey conducted 31 July - 3 August 2023. During this survey four stations had DO concentrations below 2 mg/L, eight stations had concentrations below 3.0 mg/L, and 10 stations had concentrations between 3 and 3.5 mg/L. The lowest dissolved oxygen concentration recorded during the survey was 1.69 mg/L at Station A4.

Area in square miles, Duration in days

Duration

The duration of hypoxia refers to the estimated length of time that hypoxic conditions persist in the bottom waters of Long Island Sound. Duration is estimated based on dissolved oxygen concentration readings taken at three water quality monitoring stations in the Western Sound - A3, A4, and B3.

The 2023 hypoxic event lasted an estimated 42 days, beginning on July 12, 2023 and ending on August 22, 2023.

The earliest onset of hypoxia (based on CT DEEP data only) occurred on June 25, 2002 and the latest end date was September 20, 2005. The average duration over the 33-year time series is 52 days. The longest hypoxic event was 79 days during 2008. The shortest hypoxic event was 26 days in 2017.

Hypoxic Volume

In 2019, CT DEEP and the O’Donnell lab at UCONN Marine Sciences undertook a project to develop a tool to calculate the hypoxic volume of Long Island Sound. The tool is available to the public and allows users to obtain area and volume estimates on the fly from any survey from 1991-present. The tool utilizes CT DEEP and IEC data. It can be accessed from http://merlin.dms.uconn.edu:9988/webapps/home/.

The maximum volume of water with concentrations below 3.0 mg/L occurred during the WQXXX23 survey and was XXX km3 (XXX mi2).

The interactive graph plots the maximum hypoxic volume over time.

The table compares hypoxic area and volume by survey over the 2023 season.

2023 Volume Estimates of Hypoxic Long Island Sound Bottom Waters Obtained from the UCONN HypVolApp
Area (sq km)
Volume (sq km)
CRUISE 2 mg/L 3 mg/L 4.8 mg/L 2 mg/L 3 mg/L 4.8 mg/L
WQJUN23 0 0 0 0.00 0.00 0.00
HYJUN23 0 0 0 0.00 0.00 0.00
WQJUL23 0 0 220 0.00 0.00 1.76
HYJUL23 0 0 416 0.00 0.00 11.20
WQAUG23 104 200 1104 0.51 1.82 38.05
HYAUG23 0 136 1168 0.00 1.22 37.60
WQSEP23 44 152 1092 0.00 0.54 32.29
HYSEP23 0 0 100 0.00 0.00 1.70

Frequency of Hypoxia (1994-2023)

This map illustrates how frequently the areas of Long Island Sound experience hypoxia. The colors represent the percentage of years in which hypoxic conditions have occurred in the bottom waters of Long Island Sound. The westernmost areas have experienced hypoxia almost every year since monitoring began.

The bar graph provides a deeper dive into the data (percentages) used to create the frequency map. The colors of the bars correspond to the map legend.